1. Field of the Invention
The present invention relates to an echo canceller which cancels echoes generated in telephone circuits with the echo estimate produced from the far end talker's speech signal so that customers can be free from the subjective annoyance of echo.
2. Description of the Prior Art
As is well known in the art, in telephone circuits which involve two-wire and four-wire transmission lines interconnected through hybrid transformers, a component of the incoming far end talker's speech signal transmitted through the four-wire transmission line leaks to the send path and returns as an echo to the talker, mainly due to the mismatch between the impedance of the two-wire transmission facilities looking toward the near end customer and the impedance of the balancing network in the hybrid. Customers feel the echo annoyance more as the propagation delay through the four-wire transmission line increases, because the echo returns to the far end customer with a time lag after sending the corresponding messages.
In order to overcome this problem, Dr. Sondhi et al. at Bell Laboratories has devised an echo canceller. A transversal filter receives the incoming signal and generates an echo estimate. The echo estimate is reversed in polarity and inserted into the send path. Therefore, the near end talker's speech signal passes through the send path, but the echo returning to the far end talker is counterbalanced with the echo estimate and not transmitted back to the far end talker.
The incoming signal component is transmitted through the near end echo path, accompanied by a delay. In addition, the impedance mismatching causes the waveform dispersion on the time axis. As is well known in the art, from various literature, conventional echo cancellers are four-terminal pair networks having the input and output terminals to and from the send path and the receive path. It is impossible to improve the performance, particularly the accomodated near end delay, of an echo canceller even if such four-terminal networks might be connected in cascade or in parallel. Therefore, the number of arithmetical elements of the echo canceller, in addition to those of transversal filters and their control circuit composing the echo canceller, must cover the worst near end delay. As a result, the conventional echo cancellers are obliged to be extremely large in size, compared with the other echo control devices, for instance, echo suppressors. Therefore, they are apt to have a disadvantage in both structure and cost. Attempts have been made to improve echo cancellers by utilizing semiconductor integrated circuit techniques such as LSI, but at present it is not feasible to provide such large sized LSI echo canceller because of the integration limit. The integrated circuit echo canceller would involve an extremely large number of logic gates which cannot be implemented on one chip, from the point of view of cost, performance and reliability in operation. As an example, in International Telegraph and Telephone Consultative Committee (CCITT) most of delay rates refer to the 40 millisecond (ms) echo path delay which is the sum of the abovedescribed transmission delay and the waveform dispersion time span in each country. If the total delay provided by a tapped delay circuit which makes up a transversal filter, is less than 40 ms to reduce the number of elements, it would be impossible to prevent the return of echo to the remote customers farther than 40 millisecond in end delay from the terminal station equipped with the echo canceller. On the contrary, the an LSI system which could cancel echoes with an longer end delay than 40 ms would require about 40 kilo gate arithmetical elements which is estimated to be too large to provide an LSI echo canceller chip at present.